The present invention relates to a belt for a belt drive device, and more particularly to a belt for a continuously variable belt-drive transmission for a motor vehicle.
A known continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions.
The belt comprises a plurality of metal elements adjacently arranged in the longitudinal direction of the belt, and a pair of endless carriers inserted in opposed slits of each element.
When the belt enters or leaves the pulleys, the alignment of the elements is distorted, causing misalignment of elements. Such deviations of the elements generate noise. One of the causes of the noise is that the elements strike against the pulleys. Another is that when inclination of an element to the adjacent element changes, elements bump against each other, or a gap between the adjacent elements is increased or decreased as a result of the change of the inclination so that air in the gap vibrates and oil splashes.
For example, in the case of a belt which is composed of elements each having a thickness of 2.2 mm and has a length of 589 mm, the frequency of noise is in a range between 2 and 10 kHz. The noise corresponds to the frequency of elements at a time when passing a certain point in the course of the belt. For example, when the speed of the belt is 5 m/sec, and the thickness of the elements is 2.2 mm, the frequency is 5000/2.2 =2272.72. Namely, the most commonly heard noise is 2.27 kHz. Namely, when elements have the same thickness, timing of the elements passing the certain point is constant so that sound pressure level becomes high. The sound pressure level can be decreased by changing the timing. The change of the timing can be achieved by providing a variety of the thickness of elements in a belt.
Accordingly, a belt comprising elements of two or more kinds which are different in thicknesses has been proposed in Japanese Patent Laid Open No. 61-270535 to reduce the noise. However, the prior art does not disclose differences of the thicknesses of the elements nor the combinations thereof.
On the other hand, from a production managing point of view, it is preferable to manufacture the belt with elements, the thicknesses of which are divided into a small number of types. Furthermore, in order to decrease the distortion of the belt and to prevent the carrier from excessively bending when the elements are unstable, elements are preferably formed as thin as possible provided that the strength of the element can be maintained. Accordingly, it is desirable for the difference between the thicknesses to be small.